Method and apparatus for curtain coating

ABSTRACT

An apparatus for curtain coating, particularly for high-speed curtain coating of a continuous paper web substrate ( 12 ) is provided with a hopper means ( 14 ) providing one or more liquid coating materials in the form of a free-falling curtain ( 16 ) impinging the substrate ( 12 ) at a dynamic wetting line. Edge guide elements ( 22, 24 ) are arranged on both sides supplying a wetting or auxiliary liquid. Each guide element ( 22, 24 ) has at least one contact area of its surface directed towards said curtain, which contact area has a multiplicity of grooves ( 32 ) and ribs ( 34 ) extending along the length of said edge guide elements ( 22, 24 ).

This application claims the benefit of Provisional Appl. No. 60/340,546,filed Dec. 13, 2001.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for curtaincoating of a continuously moving substrate with one or moresimultaneously applied layers of liquid coating materials, and, moreparticularly to a method and apparatus for curtain coating involving acurtain edge guide for stabilizing a coating curtain.

BACKGROUND OF THE INVENTION

Mainly in the field of manufacture of photographic papers or coatedfilms, curtain coating methods and apparatus are widely known and used.Typically a continuous web or sheets are continuously moved below acoating hopper. One or more liquid compositions are provided from ahopper arrangement in the form of a liquid curtain.

For the manufacture of photographic papers, liquid compositions are usedof relatively low viscosity, generally less than about 150 cP(centipoise), most in the range from about 5 to about 100 cP.

The manufacture of photographic papers is a tremendously difficult artrequiring extremely accurate control. The practical use of curtaincoating provides a number of difficulties coming with a need for anextremely uniform coating on the one hand and a need for coating ofsubstrates in form of a continuous web at high speeds on the other hand.

A number of problems associated with curtain coating have been addressedin the prior art and many proposals have been made to overcome suchproblems.

Besides obtaining a free-falling curtain having uniform curtaincharacteristics over its width perpendicular to the moving direction ofthe substrate, one of the most often addressed problems for coating atspeeds higher than approximately 150 m/min is the displacement ordeformation of the curtain by the air which is carried along theuncoated substrate due to friction. That air is carried along with themoving substrate to the coating point which designates the locationwhere the coating liquid first contacts the substrate. In the curtaincoating process this location has the form of a line across thesubstrate and is referred to as the dynamic wetting line. The area nearthe substrate where the air is in motion due to friction is called theboundary layer.

It has further been found important to stabilize the edge regions of thefree falling curtain to prevent narrowing of the curtain due to surfacetension of the coating liquid.

Another drawback coming with an unguided curtain is the forming of edgeregions on the coated web having greater coating thickness than theremainder of the web, which is generally undesirably and provides for aneed to cut off or remove the edge region of the coating layer or weband discard the same, as a uniform coating is normally required to meetthe expected quality standards.

Consequently, many attempts have been made to overcome the drawbacks andimprove coating performance in curtain coating of substrates.

One of the problems associated with guiding of the curtain edge isdescribed for instance in U.S. Pat. No. 5,895,687 to Kondo et al.originating from the so called “tea-pot effect” or “tea-pot phenomena”.The tea-pot phenomena may be observed with regard to a coating solutionwhich flows down along the slide surface of the coating hopper or die,and is just about to fall from the tip of a lip of the coating hopper:the coated liquid curtain layer does not fall in the vertical directiondue to a flow speed variation of the coating solution over itsthickness, causing the curtain to fall while it is curved towards thehopper. Kondo et al. propose to provide an edge guide means for thecoating curtain wherein the edge guide has a curved cross sectionalshape preferably in the form according to the tea-pot phenomena.Although it is acknowledged in U.S. Pat. No. 5,895,687 that aconventional flat plate type edge guide stabilizes a curtain layer it isdescribed as being disadvantageous and providing a thick edge layerformed on the coated substrate due to an increased contact area betweenthe flat plate type edge guide and the coating liquid.

With reference to Japanese Patent Application Open to Public InspectionNo. 99668/1989 it is stated that providing for side solutions flowing onthe edge region of the curtain would be disadvantageous because the sidesolution is intensely accumulated on each end portion of the curtain,resulting in an excessively thick layer on both edges of the coatedsubstrate.

To overcome the latter mentioned problem EP 0 740 197 A1 discloses anedge guide for a coating curtain having a dosing slot at the top and inthe region of the tip of the lip of the coating hopper for providing aside flow to reduce disturbances of the free-falling coating curtain dueto an inhomogeneous velocity profile over the width of the curtain. Withdry edge guides a problem is reported that the falling velocity of thecurtain in the edge region contacting the edge guide tends to zerobecause of the friction and adhesion of the curtain edge on the edgeguide. Consequently, an inhomogeneous coating will be obtained on thesubstrate web in the region of the curtain edge and it was oftenproposed in the prior art to involve a coating curtain being wider thanthe substrate to be coated, thus, to provide coating solution in excess.Of course, such an approach is economically unattractive because a partof the coating solution is lost and the machinery is soiled andtherefore needs frequent interruptions of the manufacturing process forcleaning operations.

The equipment proposed in EP 0 740 197 A1 comprises, in addition to thewetting guiding edge, a cutter and suction arrangement at the bottom ofthe guiding edge cutting off the outermost edge region of the curtaincontaminated with a side flow, which may comprise water or a water-basedcomposition. The outermost edge region and the side flow is removed by asuction arrangement at the bottom of the edge guide.

Further, it is known from DE 197 35 588 A1 to provide a coating curtainfrom a slot nozzle to an edge guide element, formed integrally with theslot nozzle arrangement but recessed, and further having an outlet for aside flow of the coating liquid at the top of the edge guide providingan additional flow of coating liquid on the surface of the edge guide.The inclination of the edge guide forms the falling curtain to becomenarrower at the bottom of the edge guide near the wetting line.

Further it is proposed to direct a cooling liquid through the interiorof the edge guide so that the edge guide is held at a temperature ofabout 15° C. below the temperature of the coating liquid. This measureis proposed to prevent coating composition from solidifying on thesurface of the edge guide. The edge region of the curtain is cut off bya cutter means and drained away. In one embodiment it is proposed tocurve a surface of a cutter means being in contact with the remainingcurtain parts to expand the curtain downwards of the cutting means toprevent formation of edge regions on the coated substrate having athickness different from the remainder of the coated substrate.

The disclosure of U.S. Pat. No. 5,763,013 to Devine et al. addresses theproblem that the earlier proposed arrangement for removing the edgeregion of the curtain by cutting means and a vacuum source for suckingoff the cutting off edge portion liquid provides further problems withrespect to the reliability of the manufacturing process due tosolidification causing at least partial plugging of the vacuum channels.This is reported to cause particular problems if the coating compositionincludes a setting polymer such as bone gelatin and the contact surfacesof the vacuum means have temperatures below ambient temperatures. Devineproposes providing an additional flushing liquid directly to the vacuummeans so that the liquid from the edge region of the curtain willreliably be drained from the cutting means.

EP 0 907 103 A1 proposes to provide a curtain edge guiding meanscomprising a porous layer and lubricant liquid supply means arranged inconnection with a porous layer so that the lubrication liquid isprovided over nearly the full length of the edge guiding means. Moreparticularly, it is proposed to supply the liquid along the guiding edgeat a velocity which is the same as the falling velocity of the curtainat the respective location along the guiding edge. The bottom region ofthe guiding edge is proposed to comprise a solid material easily wettedhaving a surface inclined towards the curtain edge by 1° to 5° towardsthe center of the curtain and having a suction slot at the outermostbottom having a collecting edge protruding over the surface towards thecurtain to ease removal of the lubrication liquid and outermost edgeparts of the coating curtain.

U.S. Pat. No. 5,906,865 to Ellermeier et al. reports breakage of thecoating curtain as the predominant limiting factor with respect tocoating speed and continuous operation of a curtain coater. Wetting ofedge guides or curtain holders with an auxiliary liquid is reported tobe the most proposed measure to overcome the problems originating fromturbulence in the proximity of the curtain edge. Separating devices orcutting means often proposed comprise essentially a flat cantileveredblade. This blade projects from a vacuum housing and interrupts the freefall of the curtain in the immediate vicinity of and parallel to thesubstrate to be coated. The interruption occurs just before the curtainlands. The blades need to be thin and sharp. According to a number ofproposals it is further rinsed on its upper side by cleaning liquid. Thestream of cleaning liquid rinses the liquid of the curtain edges out ofthe coating area. If the curtain edges comprise a gelatin solution, onlypart of the valuable coating solution is lost but crust may accumulateon the edge of the blade during long operation cycles. This is caused bygelatin residues. Thus, the blade becomes dull. A dull blade cannotsatisfactorily prevent a beaded coating on the edge. One of the problemscoming with the blade is a flow adhering to the surface of the blade orits underside, generally being unstable. Fundamentally, thecantilevered, sharp edged blade presents an ever-present risk to theoperators. Cleaning of the blades can result in injuries and the thinblades can be easily bent and damaged causing interruptions in thecoating process both to required cleaning and repair operations.

U.S. Pat. No. 5,906,865 proposes cutting the edge region of the curtaintogether with a wetting liquid used on a preferably flat edge guide tobe cut off by a free jet of a separating liquid, like water, and todrain off the cut edge region of the curtain and any auxiliary liquidused on the edge guide by strong vacuum source before the curtainreaches the substrate to be coated. It is pointed out that there is nowear on the jet cutting means and no risk for personnel working when thecoating process is interrupted.

According to EP 0 567 071 A1 a curtain coating method wing a slidehopper includes the supply of an auxiliary liquid in order to eliminatethe unevenness of coating thickness produced in both edge portions of acoating film which is formed by causing a free-falling coating film toimpinge on a web running continuously. The auxiliary liquid is pouredfrom a position on a distance of no more than 10 mm from a boundary linebetween guide blades and edge guides in a direction of the guide plates.The quantity of flow of the auxiliary liquid to be poured onto each ofthe guide blades is not more than 10 cc/min. A guide blade according tothis prior art has an inclination of an angle Θ=10° to 80° with respectto a slide plane in order to pour an auxiliary liquid to the side of thecoating film on a slide surface. The surface tension of the auxiliaryliquid is higher than that of the coating film so that the auxiliaryliquid is not attracted to the center of a coating film to thereby makefilm coating unstable. According to one example of this document theauxiliary liquid has a 2 cP (centipoise) viscosity, 37 dyne/cm surfacetension and 4 cc/min supplied quantity. The auxiliary liquid is pouredto the upper surface of the guide blade at a position which is at adistance of no more than 10 mm in the direction from boundary linebetween the guide blade and the edge guide.

According to EP 0 649 054 A1 a stripe internal edging method andapparatus is disclosed for curtain coating of a support with one or morelayers of the liquid coating composition using stripes of the liquidcoating composition formed at the edges of the free-falling curtain, thestripes being guided by edge guides which are positioned so that thereis an uncoated margin of support at each edge of the support. Liquid isremoved from the edges of the free-falling curtain near the point ofimpingement on the support. The apparatus and method is used especiallyfor curtain coating of very low flow rates per unit width. The apparatuscomprises flushing means for issuing liquid from the edge guide tomaintain wetting contact with the stripes. The stripe composition isgenerally an aqueous gelatin solution with appropriate surfactants addedto balance the surface tension of the stripe with a top on bottom layersof the curtain. Thickeners may also be used. Stripe viscosity isoptimally in the range of 1 to 30 cP (centipoise), especially 5 to 20cP. The flow rate of the stripe is greater than the minimum possible toachieve a stable curtain along the edge guides. The width of the stripeis at least 3 to 10 mm. A stripe air interface of at least 5 mm isformed before the stripe merges with the main body of the curtain. Thestripe is formed by means of a cavity and slot arrangement in which thestripe flows down inclined surfaces before merging with the main body ofthe curtain. Means for forming the stripe may be located on the hopperedge pad. Such a pad may be manufactured incorporating an inlet anddownwardly directed metering slot for forming the stripe. The meteringslot discharges the stripe composition at or near the lip of the hopper.Also the stripe may be guided down the edge guide by lubricating fluidintroduced through outlet and slide. The stripe fluid is providedthrough a conduit and the lubricating fluid which is preferably water isprovided through another conduit. The flow rate of the stripes isespecially approximately 1.6 cc/cm sec and the stripe viscosity is 8 cP.The surfaces of the stripe should be edged before merging with the mainbody of the curtain as otherwise the interface between the stripe andthe main body of the curtain departs significantly from vertical as thestripe flow rate is increased.

EP 0 850 696 A2 relates to a curtain coating method using an auxiliarysolution to stabilize the curtain. The auxiliary solution is to flowdown along edge guides at a flow rate between 0.3 cc/min and 3.0 cc/minfrom each side of solution injecting means. The value of surface tensionof the auxiliary solution is greater than or the same as the minimumvalue of surface tension of the coating solution to restrict the mixtureof the auxiliary solution and the coating solution to the minimum.Viscosity of the auxiliary solution is smaller than that of the coatingsolution. The auxiliary solution is either a gelatin solution of no morethan 3 percent by weight or water. The apparatus comprises slide plateshaving solution injection outlets supplying the auxiliary solution whichflows down to a boundary in the vicinity of the side plates. A flow rateof the auxiliary solution increased up to 3 cc/min makes the force ofthe curtain shrink smaller gradually, but when the amount of theauxiliary solution exceeded 3 cc/min the change on the curtaindisappeared, simply showing the thickened water layer of the auxiliarysolution. It is stated in EP 0 850 696 A2 that the more an injectingoutlet for the auxiliary solution is located at the downstream side of acurtain the less is any effect, if the injecting outlet is located at alip which is at the upstream side of the curtain or at a position abovethat the effect is greater, and where the height for supplying is thesame as the coating solution height the effect is at a maximum.Excellent coating with fewer uneven portions can be conducted using anauxiliary solution having a gelatin concentration of no more than 3percent, or water.

According to EP 0 930 530 A2 a curtain coating method and apparatus forcoating at high speed without unevenness to form uniform coatings inmulti-layer coating comprises a center line of outlets for dischargingauxiliary solution being sloped to the direction in which the coatingsolution flows down. An angle between the centerline of the outlets anda horizontal line is within 30 degrees. The outlets have a circulardiametrical section of 0.4 to 1.5 mm in diameter. The amount ofauxiliary solution discharged from each outlet is 3 to 8 cc/min. A pairof outlets for discharging the auxiliary solution is disposed in theposition along each edge part of free-falling curtain and at a fixeddistance downward from a hopper lip. The fixed distance is between 0.1and 1.5 mm. A pressure of the auxiliary solution supply is applied inthe width direction of the free-falling curtain. As an auxiliarysolution water may be used or water and methanol or a solutioncomprising water, methanol and gelatin.

U.S. Pat. No. 5,976,251 discloses edge guides for curtain coatingapparatus and delivering devices and lubricating liquid for use withcurtain coating apparatus. A dual wire edge guide is supplied withlubricating liquid without creating a stationary wave in the curtaincoating avoiding non-uniformities. Lubricating and flushing liquid issupplied through a straight horizontal conduit of constant crosssectional area with an axis lying in a plane parallel to that of thecurtain. The outlet of the conduit is in nominal contact with the dualwires. The breadth of the outlet is from about 2 to 4 mm. A land isprovided surrounding the outlet for lubricating liquid lyingsubstantially in a vertical plane perpendicular to a hopper lip,tapering downwards and terminating from about one centimeter of thehopper lip. The flow rate of the lubricating liquid may vary between 0.3and 0.5 cc/sec and the lubricating liquid can be water or a solvent forthe coating composition.

Although many approaches have been made in the prior art to overcome thedrawbacks and problems coming with the use of a curtain coating process,in particular at high coating speeds, there are still remainingdrawbacks effecting the quality and cost effectiveness of curtaincoating methods, in particular with respect to high speed curtaincoating of continuous paper web substrate.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an apparatus forcurtain coating, particularly for high-speed curtain coating of acontinuous paper web substrate, with a hopper arrangement providing oneor more liquid coating materials in the form of a free-falling curtainimpinging substrate at a dynamic wetting line wherein edge guideelements on both sides are arranged and wherein each element has atleast one contact area of its surface directed towards said curtain,which contact area has a multiplicity of grooves and ribs extendingalong the length of said edge guide elements.

Briefly stated, these and other features, objects and advantages areobtained by providing a method for curtain coating of a moving substratelike a paper web wherein a substrate moves below a hopper arrangementproviding one or more liquid coating materials in the form of afree-falling curtain impinging the substrate at a dynamic wetting linewherein improved edge guide elements are used for preventing theformation of stationary waves within the coating curtain and allowingdisturbance free provision of an auxiliary liquid as well as practicallycomplete removal of the auxiliary liquid whilst preventing the foulingof the edge guides. Further improvements obtained by the side andcurtain edge guides according to the invention are increased fallingvelocity on the curtain edge guide, the prevention of wettingdisturbances in the edge region of the coating film air entrainment,prevention from forming of coating film irregularities caused by theMarangoni-effect (a gradient of surface tension from low to highdisplaces the materials) and reducing of the Teapot-effect (a curvedtrajectory of the liquid curtain which may deviate substantially fromthe vertical trajectory) with respect to the edge guide in multi-layercoating.

In a preferred embodiment of the invention, each edge guide for acurtain coater has at least one contact area of its surface directedtowards a coating curtain, which contact area has a multiplicity ofgrooves and ribs extending along the length of the edge guide to improvewetting performance.

With the grooves and ribs the coating curtain can be held more even andcan be held better in a straight and direct line over the width of thecurtain in cross direction of moving web without making a wave-likemovement.

More preferably, the ribbed area is at least 10 mm wide, and/or thegrooves are located at a distance of about 0.5 mm from each other,whilst the top surface of the ribs are located at a distance of about0.5 mm from each other. Preferably, the essential parts of the surfacesforming a groove are inclined to each other at an angle of about 90degrees.

In preferred embodiments protrusions are arranged adjacent to the ribbedcontact area on both sides transverse to the length of the edge guide,and further protruding surface areas are arranged on the protrusionswhich protruding surface areas are essentially polished. The polishedprotruding surface areas limit the wetting area for an auxiliary liquidand/or prevent the protruding surface areas from being wetted by eitherliquid, therefore reducing the risk of spills and, thus, disturbances ofthe coating curtain.

In another preferred embodiment the edge guide has a suction opening,preferably of slot or recess configuration, as near as possible to thebottom end of the edge guide and smoothly integrated into the contactarea for removing of the wetting or auxiliary fluid, preferably locatedat the bottom end in close proximity to the substrate web.

In a further preferred embodiment each of the edge guide elements isdesigned to be made in one piece with the edge guide of a hopper slideof a curtain coater such that the surfaces of either edge guides beingdesigned to be in contact with the coating film and/or any auxiliaryliquid fit together generally without projections or recesses disturbingthe flow of the coating film and/or the auxiliary liquid causingturbulences in the fluids, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic overview showing generally a curtain coaterarrangement as known from the prior art;

FIG. 2 is a perspective view of a curtain coater edge guide arrangementfor the coating curtain;

FIG. 3 is a partial side view of the edge guide of FIG. 2 viewed ontothe ribbed area side;

FIG. 4 is a bottom view of the edge guide depicting the suction opening;

FIG. 5 is a perspective partial view of the bottom end of the edge guideas shown in FIG. 4; and

FIG. 6 is an enlarged view of FIG. 4 according to X illustrating thenature of the ribbed contact area.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the main parts of a curtain coater as known from the priorart and generally involved with an improved method and apparatusaccording to this invention. A conventional curtain coater has means,preferably in form of a backing roller (not shown), for forwardingseparate sheets or a continuous web 12 as a substrate to be coated. Theweb 12, which may comprise a paper, is forwarded along the backingroller through the curtain coater.

A hopper means 14 is located generally above the backing roller. Variousforms of hopper means 14 are known, generally providing a curtain 16 ofa coating liquid 18 free falling over a distance forwarded over a lid orany other suitable means. The coating curtain 16 is moved towards thesubstrate 12 by gravity force and impinging on the substrate web 12along a line generally perpendicular to the moving direction of thesubstrate 12. The line is generally below the lid but moving relativelyto the substrate web 12 when in motion and therefore called the dynamicwetting line.

The coating film 18, which may comprise several different layers ofliquid, is provided through one, or more in case of a multi-layercoating film 18, slot type openings 10 onto a so called slide 20 of thecoating hopper 14.

For the purpose of this application, the area of the coating film 18orientated in a direction towards the substrate web 12 is calleddownstream, whilst the coating film 18 towards the slot 10 is calledupstream.

The slide width is limited by slide edge guides 22 which generallyprovide for the width of the coating film 18. Downstream of the slideedge guide 22 and generally along the distance where the coating curtainis free falling, curtain edge guides 24 are provided to hold the coatingcurtain 16 until it impinges on the substrate web 12.

Turning to FIG. 2, a curtain edge guide 24 comprises a mounting part 26for smoothly fitting to corresponding slide edge guide 22. Preferably,the contact region depicted by 28 is formed to smoothly fit with thehopper slide edge guide 22 such that the surfaces of either edge guides22 and 24 form a planar junction without projections or recessesdisturbing the flow of the coating film and/or any auxiliary liquid sothat any turbulence in the fluids is prevented which could provide forstreaks or the like in the coating on the web 12.

Any recess provided in the slide edge guide 22 and/or the curtain edgeguide 24 for providing an auxiliary liquid between the edge guides 22and 24 and the coating film 18 run into a ribbed contact area 30 of theedge guide 24. The ribbed contact area 30 has a multiplicity of grooves32 and ribs 34 as can be seen from the enlarged view in FIG. 6, arrangedalong the curtain edge guide 24. The contact area 30 provides forguiding and holding the coating curtain 16 whilst free falling towardsthe substrate 12 and thus to keep the curtain 16 as uniform as possible.

The bottom end 40 of the curtain edge guide 24 may be formed at an angleother than perpendicular with respect to the longitudinal extension ofthe curtain edge guide 24, as depicted in FIG. 3, to meet the surface ofa substrate web 12 where the continuous web 12 is inclined to thehorizontal in the region of the dynamic wetting line.

In a preferred embodiment, the ribbed area 30 is at least 10 mm wide,more preferably about 12 mm wide. It has been found that an arrangementhaving grooves 32 located at a distance of about 0.5 mm from each otherand a top surface of the ribs 34, located at a distance of about 0.5 mmfrom each other provide an excellent wetting performance so that anyspeed difference between the curtain edge guided by the edge guide 24and the center of the curtain 16 is minimized, and thus, coating qualityis improved. It was further found that arranging the essential parts ofthe surfaces forming a groove 32 at an angle of about 90 degrees to eachother showed excellent performance.

Protrusions 36 are arranged adjacent to the ribbed contact area 30 forlimiting the wetting area of the coating liquid or any auxiliary liquidto the contact area 30. Preferably, protruding surface area 38 on theprotrusions 36 are polished to shine for preventing the surface area 38being wetted by either an auxiliary or coating liquid 18, thereforereducing the risk of spills which may cause impacts in the coatingcurtain 16, and thus, streaks in the coated web 12.

At the bottommost end 40 of the curtain edge guide 24 as shown in FIGS.4 and 5 is arranged a recess 42 for forming a suction slot connected toa channel 44 having an opening 46 for connecting to a vacuum device 60for removal of any auxiliary liquid running down the contact area 30.

The suction slot or recess 42 extends over the full width of the contactarea 30. For optimum coating performance the suction opening 42 islocated in close proximity to the substrate web 12. The suction recess42 is smoothly integrated into the contact area 30 without anyprotrusions or recesses or the like to remove the auxiliary liquidnearly completely. Threaded holes 50 may be provided for mounting of acover plate for forming the suction slot 42 with an opening of about 0.1to about 1 mm.

Where this invention has been described in terms of a preferredembodiment, the present invention can be further modified within thespirit and the scope of this disclosure. This application is thereforeintended to cover any variations, uses or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of any claims directed to this invention.

1. An apparatus for curtain coating, particularly for high-speed curtaincoating of a continuous paper web substrate, comprising a hopper meansproviding one or more liquid coating materials in the form of afree-falling curtain impinging the substrate at a dynamic wetting lineand edge guide elements on both sides of said curtain are arrangedsupplying a wetting or auxiliary liquid and wherein each element has alength along its longitudinal axis extending in a direction of flow ofsaid curtain from said hopper means and at, least one contact areadirected towards and in contact with said curtain, said contact areahaving a multiplicity of grooves and ribs extending along the length ofsaid edge guide elements thereby forming a ribbed contact area.
 2. Theapparatus according to claim 1, wherein said edge guide elements furthercomprise protrusions, said protrusions being arranged adjacent and onboth longitudinally extending sides of to the ribbed contact area. 3.The apparatus according to claim 2, wherein protruding surface areas arearranged on said protrusions and wherein said protruding surface areasare polished.
 4. The apparatus according to claim 1, wherein said edgeguide elements have a bottom end and said edge guide elements havesuction openings in the area of the bottom end of said edge guideelements for removing the wetting or auxiliary liquid.
 5. The apparatusaccording to claim 4, wherein each suction opening is arranged as arecess for forming a suction slot connected to a channel having anopening for connecting to a vacuum device for removal of any auxiliaryliquid running down said contact area.
 6. The apparatus according toclaim 5, wherein the recess extends over the full width of the contactarea.
 7. The apparatus according to claim 6, wherein the recess orsuction opening is located in close proximity to the substrate web. 8.The apparatus according to claim 5, wherein the recess is smoothlyintegrated into the contact area without any protrusions or recesses. 9.The apparatus according to claim 5, wherein threaded holes are providedin each edge guide means for mounting of a cover plate for defining adiscrete slot opening between the edge guide means and the cover plate.10. The apparatus according to claim 9, wherein said discrete slotopening has a size of about 0.1 to about 1 mm.
 11. The apparatusaccording to claim 1, wherein each of said edge guide elements isdesigned to be made in one piece with an edge guide of said hoppermeans, and wherein each said edge guide element and said edge guide fittogether generally without projections or recesses which wouldturbulently disturb the flow of the coating and/or the wetting liquid.12. The apparatus according to claim 1, wherein the ribbed area is at 10mm wide, and/or the grooves are located at a distance of about 0.5 mmfrom each other.
 13. The apparatus according to claim 12, wherein theribs have top surfaces and the top surfaces of the ribs are located at adistance of about 0.5 mm from each other.
 14. The apparatus according toclaim 12, wherein the ribbed area is at least about 12 mm wide.
 15. Theapparatus according to claim 1, wherein surfaces forming each saidgroove are inclined to each other at an angle of about 90 degrees. 16.The apparatus according to claim 1, wherein each edge guide means has abottom end and wherein the bottom end of each edge guide means is formedat an angle other than perpendicular with respect to the longitudinalaxis of the edge guide elements.